1. The Field of the Invention
The invention is in the field of endodontics and endodontic instruments for use in preparing root canals to receive a filling material such as gutta percha. More particularly, the invention is in the field of processes for manufacturing endodontic instruments.
2. The Relevant Technology
When a root canal of a living tooth becomes infected or abscessed, discomfort and, in many cases, severe pain can result. In the early days of dentistry the only solution was to pull the tooth. More recently, however, dental practitioners have learned to successfully remove the pulp material forming the nerve of the tooth that has become infected and, after careful preparation of the canal that contained the nerve material, refill the canal with an inert filling material, such as gutta percha, permitting a patient to retain the tooth.
In order to achieve a successful root canal restoration, the dental practitioner must carefully and, as completely as possible, remove the infected pulp material of the tooth to prevent continued or future infection of surrounding tissues. The removal process typically includes shaping of the root canal so that it can be effectively and successfully filled and sealed with an inert material to eliminate the possibility of further infection occurring within the cleaned and shaped root canal.
Cleaning and shaping the root canal in preparation to filling with a material such as gutta percha is achieved by the use of metal files that include cutting surfaces for removing tissue in the root canal. The cutting surfaces are typically formed by helical flutes formed in the file. One or more helical cutting surfaces may be provided, which may be axially spaced as desired.
Some existing endodontic instruments and manufacturing methods are described in U.S. Pat. No. 4,934,934, U.S. Pat. No. 5,653,590, U.S. Pat. No. 5,762,541, and U.S. Pat. No. 6,890,134.
Since root canals are seldom straight, often having bends and twists, at least some endodontic files are advantageously flexible. Currently preferred materials of construction include stainless steel, and more recently, nickel-titanium (Ni—Ti) alloys. Such materials, especially Ni—Ti alloys, exhibit good flexibility, resilience and strength, and are not likely to fail during use. Flexibility and strength are important to avoid file breakage during the cleaning process.
Endodontic instruments may be designed to be manually manipulated or to be fitted to a powered handpiece that provides rotation of the file during its use. An endodontic instrument that is intended for hand use is typically provided with an enlarged diameter plastic handle attached to the proximal end of the instrument, configured for easy manipulation between the thumb and forefinger of the dental practitioner. An instrument intended for use with a powered handpiece has a stem at the instrument proximal end configured to be removably received within a chuck of the powered handpiece, by which the instrument may then be rotated as desired by a dental practitioner.
One current method of manufacturing existing endodontic files is by a grinding operation. In the grinding operation, a metallic (typically a titanium alloy) rod is advanced past a rotating grinding wheel at a relatively slow feed rate. The depth of cut may be varied along the length of the rod in order to produce a tapered endodontic file. Such a method is disclosed in U.S. Pat. No. 5,762,541.
Tapering and grinding the rod in this way requires complex and precise machining equipment with many moving parts to perform the grinding, rotating, and tapering of the rod. The grinding apparatus can also easily become clogged with material removed from the metallic rod. The method is quite complex and relatively expensive. In addition, methods that include grinding of the rod can result in the formation of microcracks within the metal rod, which problem is particularly common when forming instruments from superelastic (e.g., Ni—Ti) metal alloy materials.
U.S. Pat. No. 6,890,134 teaches a method of manufacturing an endodontic file by use of a milling cutter tool. Although such a method may provide some advantages over grinding methods, the method as taught in U.S. Pat. No. 6,890,134 includes other disadvantages. For example, the method discloses using a single milling cutter tool to cut material from a metallic blank, forming a single helical flute into the metallic blank. It is often desired to manufacture endodontic instruments including multiple flutes, which would require refeeding of the metallic rod through the milling apparatus two or more times. In addition to the increased manufacturing time and cost, it can be very difficult to achieve high quality and precision in the cutting of additional helical flutes when refeeding of the metallic rod is required. The result may be increased rejection rates for manufactured parts and increased costs.
It would be an improvement in the art to provide an alternative method of manufacture capable of producing tapered endodontic instruments having multiple helical flutes at a reasonable cost.